The invention relates to a frequency modulation (FM) transmitter for radio transmission of frequency modulated stereo audio signals.
FM transmitters for radio transmission of frequency modulated stereo audio signals have been used. A typical FM transmitter has an arrangement as shown in FIG. 2.
As seen in FIG. 2, the transmitter includes:
a right audio section 10 having a pre-emphasis circuit 11, a volume 12, a limiter 13, a low-pass filter 14, and a muting circuit 15;
a left audio section 20 having a pre-emphasis circuit 21, a volume 22, a limiter 23, a low-pass filter 24, and a muting circuit 25;
a stereo modulator section 30 having an audio amplifier 31 for amplifying audio signals received from the audio sections 10 and 20, an oscillator circuit 32 coupled with an external quartz oscillator Xosc1 (having a fundamental frequency of 38 kHz) to generate 38 kHz signals, a multiplexer 33 for switching between right and left amplified 38 kHz audio signals received from the oscillator circuit 32, and a radio frequency (RF) amplifier circuit 34;
a modulation level adjust circuit 41 for regulating the output level of the multiplexer 33;
a pilot level adjust 42 for regulating the level of 19 kHz pilot signal obtained by frequency dividing the 38 kHz output of the oscillator circuit 32;
a mixer circuit 43 for mixing the regulated output signals of the modulation level adjust circuit 41 with the output of the pilot level adjust circuit 42 to generate a composite signal;
a PLL frequency synthesizer 44 coupled to an external quartz oscillator Xosc2 (having a fundamental frequency of 7.2 MHz) and with the RF amplifier circuit 34 to receive the RF output thereof to generate a frequency control signal;
an oscillation control section 47 having a low-pass filter 45 for filtering the low frequency components of the output of the PLL frequency synthesizer 44;
a mixing circuit 46 for mixing the output of the PLL frequency synthesizer 44 with the composite signal received from the mixer circuit 43 to generate a frequency modulation signal;
a frequency modulation circuit 48 controlled by the output signal of the frequency control section 47; and
an RF output level regulation circuit 49 for regulating the RF output of the RF amplifier circuit 34.
It is noted that the stereo modulator section 30 and the PLL synthesizer 44 are themselves provided in the form of integrated circuits. The PLL synthesizer 44 is represented by a single block in FIG. 2, but it actually comprises several components such as a frequency divider, a phase comparator, and program counter.
The PLL synthesizer 44 frequency divides the signal generated by the quartz oscillator Xosc2 (having 7.2 MHz) into several different frequencies and supplies its output to an input end of a phase comparator for use as reference frequency signals. The RF frequency signal generated by the oscillation modulator circuit 48 is also frequency divided by a program counter, which are supplied to the other input end of the phase comparator for comparison with the reference frequency signals. Upon comparison of the input signals, the phase comparator provides its outputs to the oscillation control section 47, which determines RF frequencies based on the frequency divided frequencies and the reference frequencies.
The PLL frequency synthesizer 44 is used to generate reference frequencies, for example, 100 kHz, 50 kHz, 25 kHz, 10 kHz, 9 kHz, 5 kHz, and 1 kHz for radio broadcasting. In this case, an appropriate external quartz oscillator Xosc2 is one having a fundamental frequency of 7.2 MHz.
As discussed above, prior art FM transmitters have been constructed as a composite of different types of elements such as audio sections 10 and 20, a stereo modulation section 30, a PLL frequency synthesizer 44, an oscillation controller 47, and a frequency modulation circuit 48. Of these, the stereo modulator 30 and the PLL frequency synthesizer 44 are formed into integrated circuits.
As a result, these components are arranged in a complex configuration and require complex wiring thereof when these elements are integrated to form an FM transmitter.
In addition, the stereo modulator section 30 and the PLL frequency synthesizer 44, built in the integrated circuit configurations, utilize different quartz oscillators Xosc1 and Xosc2 having different fundamental frequencies (38 kHz and 7.2 MHz), respectively, for their intended purposes. The use of such different quartz oscillators in one transmitter inevitably results in an extra manufacturing cost.
In accordance with one aspect of the invention, a frequency modulating (FM) transmitter includes:
a reference frequency generator for generating a reference frequency;
a reference frequency divider for frequency dividing the reference frequency;
a stereo modulation circuit for frequency modulating a right audio signal and a left audio signal by using one output of the reference frequency divider to supply resultant stereo modulated signals as FM radio signals;
an oscillator circuit for generating carrier waves to transmit the FM signals received from the stereo modulation circuit;
a program counter for frequency dividing the carrier waves into variable frequency components; and
a PLL frequency synthesizer which has a phase comparator circuit for comparing the variable frequency components output from the program counter with another output of the reference frequency divider to provide at an output end of the PLL frequency synthesizer a control signal for controlling the oscillator circuit.
In this arrangement, since the FM transmitter may generate various frequency signals for both the stereo modulation and frequency comparison in the PLL frequency synthesizer by means of a single oscillator, the resulting FM transmitter has most of the elements integrated in one chip, which implies that the transmitter has far smaller dimensions and a simpler structure in number and arrangement of elements than conventional transmitters, and hence offers increased reliability and reduced manufacturing cost. Further, the program counter may be constructed such that the frequency division ratios of the program counter and the modulation level of the stereo modulator circuit are controllable by external means. It is then possible to vary the carrier frequency of the FM transmission wave as needed and to set the FM modulation at a characteristically optimum level.
The frequency of the reference frequency generator may be chosen to be 7.6 MHz or an integral multiple or fractional frequencies of 7.6 MHz obtained by dividing 7.6 MHz by integers (hereinafter referred to as integer fractions). It should be noted that this choice of the fundamental frequency allows provision of not only commonly used 38 kHz and 19 kHz Stereo modulation frequencies through the frequency divisions but also FM radio frequencies which are close to conventional frequencies by the same quartz oscillator.
In accordance with another aspect of the invention, a frequency modulating (FM) transmitter includes:
a reference frequency generator for generating a reference frequency;
a reference frequency divider for frequency dividing the reference frequency;
a stereo modulation circuit for frequency modulating audio signals by using one output of the reference frequency divider to supply resultant stereo modulated signals as FM signals;
an oscillator circuit for generating carrier waves to transmit the FM signals;
a program counter for frequency dividing the carrier waves into variable frequency components; and
a PLL frequency synthesizer which has a phase comparator circuit for comparing the variable frequency components output from the program counter with another output of the reference frequency divider.
In accordance with another aspect of the invention, a method of generating an FM signal includes:
generating a reference frequency;
dividing the reference frequency using a reference frequency divider;
frequency modulating a right audio signal and a left audio signal using one output of the reference frequency divider to supply FM signals;
generating carrier waves to transmit the FM signals using an oscillator circuit;
dividing the carrier waves into variable frequency components; and
comparing the variable frequency components with another output of the reference frequency divider using a phase comparator circuit in a PLL frequency synthesizer.